Not Applicable
The present invention generally relates to the field of optical communication systems, and in particular to a method and optical device for anti-PDL beam swapping.
In optical wavelength division multiplexed (WDM) communication systems, a single optical waveguide simultaneously carries many different communication channels in light of different wavelengths. In general, each communication channel is assigned a nominal centre wavelength, and the channel spacing, or separation, is defined for the network. The closer the channel spacing, the greater the number of channels that may be transmitted over an optical fiber of the network. The International Telecommunications Union has proposed Dense WDM (DWDM) network standards with optical signal channels having a frequency separations of 25, 50, and 100 GHz (equivalent to a wavelength separation of about 0.2, 0.4 and 0.8 nm, respectively). Lower frequency separations are envisioned. Accordingly, the performance requirements for DWDM networks (such as those for bandwidth, cross talk, polarization dependent loss, polarization mode dispersion, and insertion loss) are becoming more stringent.
Unfortunately, many of the optical components used DWDM communication systems are polarization sensitive. For example, the diffraction gratings used in the dynamic gain equalizer (DGE), configurable optical add/drop multiplexer (COADM), and wavelength blocker (WB) taught in U.S. patent application Nos. 200020067887, 20020009257, (Attorney Doc. No. 10-510), incorporated herein by reference, are typically polarization sensitive. Accordingly, a front-end unit that provides light having a predetermined polarization is included in these designs, and others. In general, the front-end unit includes one or more polarization beamsplitters and one or more half-waveplates. The polarization beamsplitters split an input optical signal into two orthogonally polarized sub-beams of light, while the half-wave plate(s) alters the polarization of at least one of the beams so that both sub-beams have a same polarization state. A disadvantage of polarization beamsplitters is that they typically introduce a difference in optical path length for the two spatially separated sub-beams of light, and thus introduce polarization mode dispersion (PMD).
In addition to the front-end unit, these designs also include a liquid crystal (LC) array or a micro-electro-mechanical systems (MEMS) array. LC and MEMS arrays have played an important part in optical communication systems, since they are designed to simultaneously switch/modify spatially resolved portions of the optical signal independently from each other and because they are designed to be compact, have a low power consumption, and be mass produced at a low cost. Unfortunately, both LC cells and MEMS components often exhibit local spatial dependencies. For example, the retardance provided by an addressable region of a LC modulator is not necessarily uniform over the entire region, while individual reflective MEMS elements often exhibit an undesired curvature at the outer regions thereof. Since each element of the above mentioned arrays exhibits performance variations for each sub-beam of light transmitted thereto, each sub-beam will typically be altered to a different extent. This introduces polarization dependent loss (PDL).
It is an object of the instant invention to provide a method and device that lessens or obviates both PMD and PDL.
It is an object of this invention to provide an optical device that has the same optical path length for two split sub-beams of light propagating therethrough.
It is an object of this invention to provide an optical device that uses the same region of a MEMS or LC or other modulator device for two split sub-beams of light propagating thereto.
The instant invention relates to a method and apparatus that reduces or eliminates PDL and PMD in optical devices that use a polarization diversity unit to produce two spatially separated beams of light having a same polarization state. In particular, the invention relates to a method and apparatus using a beam swapping element disposed for receiving the two beams of light and for redirecting the two beams of light to a same overlapping area of a modulator, where they are modified and reflected back to the polarization diversity unit. The beam swapping element is designed and positioned such that the two beams of light swap positions upon reflection from the modulator.
In accordance with the invention there is provided a method of beam swapping comprising the steps of: providing an input optical signal; spatially separating the input optical signal into first and second beams of light having orthogonal polarizations; rotating the polarization of at least one of the first and second beams of light so that the first and second beams have a same polarization state; routing the first beam of light to a first surface of a beam swapping element and the second beam of light to a second other surface of the beam swapping element, the first and second surfaces disposed to redirect the first and second beams of light, respectively, to a same location; modifying the first and second beams substantially at the same location and reflecting them in a backwards directions such that the first beam of light is transmitted to the second surface of the beam swapping element and the second beam of light is transmitted to the first surface of the beam swapping element; and rotating the polarization of at least one of the first and second beams of light transmitted such that they have orthogonal polarization states and recombining them to form an output optical signal.
In accordance with the invention there is further provided an optical beam swapping device comprising: a polarization diversity unit for receiving an input optical signal and producing a first beam of light and a second beam of light therefrom, the first and second beams of light having a same polarization state; a beam swapping element disposed for receiving the first and second beams of light and for redirecting the two beams of light to a same point; a modulator disposed substantially about the same point for modifying the first and second beams of light; and a reflective surface for reflecting the two modified beams of light back to the polarization diversity unit where they are combined to form an output optical signal, wherein each of the first and second beams of light traces out the other""s optical path in reverse.
In accordance with the instant invention there is provided an optical beam swapping device comprising: a polarization diversity unit for receiving an input optical signal and producing a first and a second beam of light therefrom; a beam swapping element disposed for receiving the first and second beams of light and for redirecting the two beams of light to a same point; an optical component having a local spatial dependence on an optical property disposed substantially at the same point; and a reflective surface disposed for receiving the two beams of light directed to the same point and reflecting them back to the polarization diversity unit where they are combined to form an output optical signal, wherein each of the first and second beams of light is reflected in a backward propagating direction along an optical path that the other beam of light followed to the reflective surface in a forward propagating direction.
In accordance with the instant invention there is provided an optical device comprising: polarization diversity means for providing first and second spatially separated beams of light from a single input beam of light; reflective means disposed for receiving the first and second beams of light and redirecting them back to the polarization diversity means where they are recombined into a single output beam of light; and a beam swapping element optically disposed between the polarization diversity means and the reflecting means for forcing the first and second beams of light to swap optical paths upon reflection from the reflecting means.
Advantageously, providing a beam swapping element forces each sub-beam of light produced by the birefringent crystal to trace out the other""s path through the module in order to minimize the difference in retardance and/or optical path experienced by each of the beams.